The present invention relates to systems for determining whether a reticle is defective. More particularly, the present invention relates to systems and methods which identify reticle defects that arise at some time after a reticle is xe2x80x9cqualifiedxe2x80x9d as being suitable for use.
A normal reticle or photomask is an optical element containing transparent and opaque regions which together define the pattern of coplanar features in an electronic device such as an integrated circuit. A phase shift reticle, which is also well known in the art, may further include graded regions (with varying thickness) that cause a phase shift of the transmitted light. In order to learn more about phase shift reticles, reference may be made to book authored by Van Zant, Peter, entitled xe2x80x9cMicrochip Fabricationxe2x80x9d McGraw-Hill, 1997, which is incorporated herein by reference for all purposes. Reticles are used during photolithography to define masks which protect specified regions of a semiconductor wafer from etching, ion implantation, or other fabrication process. For many modern integrated circuit designs, a reticle""s features are between about 4 and about 20 times larger than the corresponding feature size of the mask on the wafer.
Reticles are typically made from a transparent medium such as a borosilicate glass or quartz plate on which is deposited an opaque pattern of chromium or other suitable material. The reticle pattern may be created by a laser or an e-beam direct write technique, for example, both of which are widely used in the art. The reticle is framed and covered by a pellicle which is a thin layer of an optically neutral material such as a polymer attached to the frame. Typically, an adhesive is used to affix the pellicle to the frame. Once in place, the pellicle (positioned about 6 mm from the reticle) protects the reticle from dirt or dust particles in the environment. Such particles may deposit on the pellicle but do not affect the reticle""s image because the pellicle is located beyond the focal plane of the reticle.
During the normal course of the reticle""s life, however, defects can be introduced into the reticle. For example, particles may be present but hidden (on the chromium region for example) when the reticle is initially formed. Over the course of time, some of these particles may migrate onto the transparent regions where they degrade the image quality. In another example, defects may be introduced into the reticle by xe2x80x9cflakingxe2x80x9d of the frame or the adhesive material that affixes the pellicle to the frame.
In yet another example, an electrostatic discharge (ESD) generated by a stepper apparatus employed during conventional photolithography may damage the opaque regions of the reticle.
If a reticle becomes defective due to one of the above mechanisms, for example, it may have a very negative impact on the yield of an IC fabrication facility. For example, a particle spanning two opaque lines on a reticle may result in shorting between adjacent metal or polysilicon lines. Other reticle defects may cause more subtle defects that can not easily be detected and may not be manifested until the ICs are in the customers"" hands. Undetected, such defects can cost a facility many millions of dollars and potential embarrassment. Thus, many IC manufactures periodically image or otherwise test their reticles to ensure that they are not defective.
FIG. 1A is an idealized representation of an actual xe2x80x9cdarkfieldxe2x80x9d image 10 of a reticle obtained by scanning a light beam onto the reticle and monitoring light scattered therefrom. In the actual image, various regions of the image have varying shades of gray. In FIG. 1A, the various shades of gray could not be accurately depicted, so the contrast between features is exaggerated in some cases and reduced in other cases.
Image 10 of the reticle has a dark area 14, a bright area 16, and a very dark area 12. Areas 14 and 16 have certain relatively bright repetitive features created by light scattering off of valid repetitive features on the reticle surface. For example, dark area 14 includes vertical lines 22 created by some repetitive feature on the reticle. Such image patterns created by valid structures may fool a detector into believing that they constitute defects. Therefore these features are sometimes referred to herein as xe2x80x9cfalse defects.xe2x80x9d In addition to vertical lines 22, dark area 14 also has a random bright spot 18 indicative of a reticle defect (hereinafter referred to as a xe2x80x9creal defectxe2x80x9d), which may be caused by an electrostatic discharge (ESD) for example.
Bright area 16 receives its brightness from bright bands 20 which are light scattered off of valid die features (more examples xe2x80x9cfalse defectsxe2x80x9d). Very dark area 12 contains very little scattered light and no bright spots that would represent real or false defects on the reticle.
As should be apparent from a study of FIG. 1A, various real and false defects may appear in an image. Obviously, a test system must be able to separate the real from the false. Traditionally, this has been accomplished by employing a xe2x80x9cdie-to-diexe2x80x9d comparison which may be carried out in KLA 301 or 351 Reticle Inspection Tool, commercially available from KLA-Tencor of San Jose, Calif.
In systems employing the die-to-die approach, the images of two supposedly identical patterns on a reticle are compared. Note that many reticles contain the patterns of multiple identical die, collectively referred to as a field. Images of two or more of these individual die patterns in a field are compared by optically overlaying the patterns. Such comparisons will screen the false defects because they will be found on the images of both die. Real defects presumably occur randomly and therefore appear only on a single die. Thus, a comparison of two die pattern images will normally find a real defect on only a single die pattern. Thus, the imaging system will flag bright spots appearing on only a single image as real defects.
FIG. 1B shows some significant components of a scattering or xe2x80x9cdarkfieldxe2x80x9d detecting assembly 50 that may be employed to scan a reticle surface and generate an image of the reticle or die pattern. An incident beam 56 generated by an illuminating source 52, e.g., a laser, is directed at a portion of a reticle surface 54. Incident beam 56 travels along an incident axis 70 and perpendicular to an axis 72. First and second detectors 64 and 68, positioned at an oblique angle, e.g., 45xc2x0, with respect to the incident axis 70, detect a first and second scattered energy signals 58 and 60, respectively, from reticle portion 54 after the scattered energy signals pass through filters 62 and 66.
During a typical inspection process of reticle portion 54, illuminating source 52 directs incident beam 56 to strike reticle portion 54 and a resulting scattered light signal is detected by first and second detectors 64 and 68. A defect residing at reticle portion 54 may, therefore, be flagged, if the intensity of the detected light signal is equal to or exceeds a predetermined threshold signal intensity. If, however, the intensity of the scattered energy signal detected is less than a predetermined threshold signal intensity, then reticle portion 54 is considered to be free of defects. Typically, the source and detectors are moved in a rasterized fashion to generate an image of the entire reticle.
During a die-to-die comparison in the same reticle, it may be difficult to discriminate between false defects and true defects. This is because there may be subtle differences between the dies that are not necessarily true printable defects. For example, small differences in feature width may fall within acceptable tolerances but still show up as defects on die-to-die comparisons. Further, some reticles contain a pattern for a single die only. Obviously, in such cases die-to-die reticle inspection can not be implemented.
What is needed is an improved inspection system that rapidly and inexpensively determines whether a defect has appeared in a reticle.
The present invention provides a method of inspecting a reticle for defects that occur over time. The invention accomplishes this by generating and storing a xe2x80x9cbaselinexe2x80x9d image of the reticle and then periodically generating a xe2x80x9ccurrentxe2x80x9d image of the reticle and comparing the current and baseline images. The baseline image is taken at a time when the reticle is known to be acceptable. Often this is when the reticle has been xe2x80x9cqualifiedxe2x80x9d by an optical test or a die fabricated by reticle has been electrically tested. Because the comparison relies upon images of the exact same portions of the reticle, the problems inherent in die-to-die techniques are avoided.
In some cases, the methods of this invention may be characterized by the following sequence: (a) providing a baseline image of the reticle which was created while the reticle was qualified as being of acceptable quality; (b) generating a current image of the reticle (preferably in the same manner as the baseline image); and (c) comparing the baseline and current images to identify any new defects that may have arisen in the time between when the baseline image was created and when the current image is generated.
The baseline and current images may be obtained by scanning the surface of the reticle with light from an illumination source. For each region considered in the scan, the method may involve (i) illuminating a region of the reticle by an incident beam generated by an illuminating source; (ii) detecting a scattered energy distribution from the region of the reticle by a detector; and (iii) recording the scattered energy distribution from the region of the reticle.
The baseline and current images may be compared by a process involving first determining whether intensity of scattered radiation at a first location of the current image is greater than a defined threshold; and if so, then determining whether a corresponding region of the baseline image also contains scattered radiation of substantially the same intensity. The initial comparison of the current image with a threshold speeds the overall comparison. If a particular portion of the current image does not exceed the threshold, then no significant scattering occurred there which means that the no defect resides therexe2x80x94regardless of any comparison to the baseline image. If a direct comparison of the baseline and current images is necessary, then those regions where the intensity value of the current image significantly exceeds the corresponding intensity value of the baseline image are deemed to contain a defect.
Various techniques may be employed to facilitate the general methods of this invention. For example, the baseline image may be compacted to reduce the quantity of stored data for portions of the image where the intensity of the scattered radiation does not exceed a defined threshold. In a preferred embodiment, compacting includes the following: (i) segmenting the baseline image into regions of the reticle; and (ii) removing data from the baseline image for those regions of the reticle where the intensity of the scattered radiation does not exceed the defined threshold.
In addition, multiple imaging algorithms may be employed to image various regions of the reticle under evaluation. For each region, a best algorithm is selected. This best algorithm is better able to discriminate between real and false defects than any other algorithms. In one embodiment, a suitable method includes the following sequence: (a) providing the reticle to be inspected; (b) generating data specifying intensity of radiation scattered from the reticle as a function of location on the reticle; (c) defining a first portion of the data which was derived from a first region on the reticle; (d) applying a plurality of imaging algorithms to the first portion of the data; (e) selecting a first imaging algorithm from among the plurality of imaging algorithms based upon ability to suppress scattered radiation from valid features on first region of the reticle; and (f) associating the first imaging algorithm with the first portion of data in the baseline image, such that during the subsequent inspections of the reticle the first imaging algorithm is applied to the first portion of the data to provide an image of the first region of the reticle. Generally, the method will also involve storing in memory an association of the first imaging algorithm with the first region of the reticle. In addition, the various imaging algorithms may be ranked according to ability to suppress scattered radiation from valid features on the reticle.
In some embodiments, the image data is generated in multiple passes. In such embodiments, the process may involve the following: (I) carrying out a first scan of the reticle under a first apparatus setting for determining intensity of radiation scattered from the reticle as a function of location on the reticle; (ii) carrying out a second scan of the reticle under a second apparatus setting which are different from the first apparatus setting; and (iii) selecting an apparatus setting based upon ability to suppress scattered radiation from valid features on the reticle. Preferably, (iii) is performed for each of a plurality of regions of the reticle. Thus, the system will associate a selected apparatus setting with one or more regions of the reticle, such that during the subsequent inspections of the reticle data from the selected apparatus setting may be used to image the one or more regions.
These and other advantages of the present invention will be described in more detail below in the detailed description of the invention and in conjunction with the following figures.